Basic correction:

Dear Bob,

I am very sorry to hear that you were very bothered by the noise from my house these days. I apologize for your discomfort and I will fix the problem as soon as I can.

A water pipe in my bathroom was leaking last week. Because it is underneath the concrete floor, in order to inspect it, I had to drill some holes on the floor with my drill, which caused a lot of noise at that time. Luckily, the source of the problem was found yesterday.

In order to finish this repair work efficiently, I have contacted a local construction company and they will send some professional plumbers to fix it. As we discussed, some advanced engineering methods would be taken to avoid too much cutting and drilling works which could mainly cause noise.

I think all repairing work should be finished by this afternoon and there should be no more noise to trouble you any longer. Please accept my sincere apologies regarding this issue.

Best regards,

Guangyu

How I would write it:

Dear Bob,

I am truly sorry that you were bothered by the noise from my house. A pipe in my bathroom was leaking last week, so in order to inspect it, I had to drill some holes through the concrete floor. Luckily, the source of the problem was found yesterday and I contacted a local construction company to take care of it professionally. Their expert repair work should keep noise at a minimum and everything should be finished by this afternoon. Thank you for your patience, and my sincere apologies again for the inconvenience.

Best regards,
Guangyu